Light emitting diodes (LEDs) are attractive candidates for replacing conventional light sources such as incandescent lamps and fluorescent light sources. The LEDs have higher light conversion efficiencies and longer lifetimes. Unfortunately, LEDs produce light in a relatively narrow spectral band. Hence, to produce a light source having an arbitrary color, a compound light source having multiple LEDs is typically utilized. For example, an LED-based light source that provides an emission that is perceived as matching a particular color can be constructed by combining light from red, green, and blue emitting LEDs. The ratio of the intensities of the various colors sets the color of the light as perceived by a human observer.
The intensity of the LED can be varied by varying the drive current or the duty factor. In duty factor schemes, the LED is pulsed on a time scale that is too fast to be seen by a human observer. During each cycle, the LED is on for some fraction of the cycle time. Since the observer's eye integrates the light received over a time period that is long compared to the cycle time, the observer “sees” a light source whose intensity is proportional to the duty factor, i.e., the ratio of the time the LED is turned on to the time the LED is turned off. The intensity is a linear function of the duty factor, and hence, the control system is relatively simple.
Unfortunately, the output of the individual LEDs varies with age. Hence, a light source that provides the desired color at one point in time will exhibit a color shift when the conditions change or the device ages. To avoid these shifts, some form of feedback system is typically incorporated in the light source to vary the driving conditions of the individual LEDs such that the output spectrum remains at the design value in spite of the variability in the component LEDs used in the light source.
Typically, a prior art light source having a feedback system for maintaining the color perceived by a human observer at a predetermined hue is constructed from a plurality of LEDs that emit light at different wavelengths. Each LED is viewed by a photodetector that includes an appropriate filter that is used to measure the light that is generated by that LED. The output of the photodetector is compared to a target value to generate an error signal that is used to adjust the light output of the corresponding LED either by changing the drive current or the duty factor.
Unfortunately, such feedback systems substantially increase the cost and complexity of the light source. The cost of the photodiodes used to monitor the LEDs is similar to that of the LEDs themselves. Furthermore, the photodiodes must be positioned such that each photodiode views only one LED. If each LED in the light source emits a distinct color, color filters can be utilized to isolate the various photodiodes. However, these filters add to the cost of the light source. In addition, many LED-based light sources of interest utilize a number of LEDs of each color to provide increased light output. In such systems, additional light isolation systems must be utilized that further increase the cost and complexity of the systems.